Same watts at 8 and 4 ohms?

With NAD it’s their power supplies and the way they rate. They use 4 Ohm all channels driven over full 20hz-20khz spectrum at certain distortion. They should also show a dynamic rating which will vary by Ohm. For the C388 it’s 250 8 Ohm, 350 4 Ohm and 400 2 Ohm. 4 Ohm speakers need more power from the amplifier than 8 Ohm, the main thing is good power supply in the amp. 

Its one of the more useless things to know. What matters most with amps isn’t the power of the amp but the sensitivity, and to a lesser extent the impedance, of the speakers. Because if you have speakers with at least 90dB sensitivity and the impedance isn’t whack then you will be fine with just about any amp of around 50 watts. If your speakers are 95dB or greater and never go below 4 ohms you will be fine with virtually any amp of any power. But if you go below 90dB, to say 85dB, and if they also drop below 4 ohms, then good luck! So its the speakers, not the amp.

Still, marketing departments find it easier to sell amps by talking about watts. A lot of audiophiles get sucked into it. Don’t. Here’s why its BS and why what I said is the most important technical thing you can learn about amps: sound and power are logarithmic.

What this means is, what you hear when going from 90dB to 93dB is only a tiny little bit louder. But that same 3dB requires TWICE as much power! Going from 90dB to 100dB sounds about twice as loud, but takes TEN TIMES as much power.

So what happens is most of the time, the vast vast majority of the time, even when listening quite loud, your amp is putting out fractions of a watt. Only when you turn it up real loud, then it goes from one watt to ten to a hundred in nothing flat. If you screwed up and bought the 85dB speaker good luck, you now need a thousand watts.

So you can forget power. Its not power that makes a speaker sound good anyway. I can prove this to you all day long going back and forth between 20 watt tube amps and 200 watt solid state. All. Day. Long. And no its not even tubes and SS, because there’s low watt SS amps that sound more powerful than high watt tube amps. Not many. But they’re there. Point is its not about watts. Its about sound quality.